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\begin{document}

\IBVShead{5864}{1 December 2008}

\IBVStitletl{Variable stars in the field of the open cluster NGC~457}

\IBVSauth{Maciejewski,~G.$^{1}$; Bukowiecki,~{\L}.$^{1}$; Bro\.zek,~T.$^{1}$; Georgiev,~Ts.$^{2}$; Boeva,~S.$^{2}$;\\ Kacharov,~N.$^{3}$; Mihov,~B.$^{2}$; Latev,~G.$^{3}$; Ovcharov,~E.$^{3}$; Valcheva,~A.$^{3}$}

\IBVSinst{Centrum Astronomii Uniwersytetu Miko{\l}aja Kopernika, Gagarina 11, Pl-87100 Toru\'n, Poland;\\ e-mail: {\tt gm@astri.uni.torun.pl}}

\IBVSinst{Institute of Astronomy, Bulgarian Academy of Sciences, 72 Tsarigradsko Chausse Blvd., 1784 Sofia, Bulgaria; \\ e-mail: {\tt tsgeorg@astro.bas.bg}}

\IBVSinst{Department of Astronomy, University of Sofia, 5 James Baurchier Str., 1164 Sofia, Bulgaria}


\SIMBADobjAlias{NGC 457}{C 0115+580}
\SIMBADobj{V466 Cas}
\SIMBADobj{V765 Cas}
\WEBDAobj{NGC 457}


\IBVSabs{We presents results of the search for variable stars in the field of the}
\IBVSabs{young open cluster NGC 457. Twenty nine new and two known variables were}
\IBVSabs{detected and analysed. Six of them were found to be unquestionable}
\IBVSabs{or likely cluster members.}

\begintext

According to the WEBDA\footnote{http://www.univie.ac.at/webda/} data base (Mermilliod 1996), the open cluster NGC~457 is known as a relatively young stellar system of $\log (age) = 7.324$, the reddening $E(B-V)=0.472$ mag, and the distance of 2.43~kpc. Its apparent diameter was estimated to be 20' (Dias et al.~2002). No search for variable stars in the cluster has been performed to date.

The field of NGC~457 was searched for variable stars in $B$ and $V$ bands with two Schmidt telescopes. The first campaign was performed between 2007 November and 2008 April with the 90/180 cm Schmidt-Cassegrain Telescope (TSC90) of the Nicolaus Copernicus University Astronomical Observatory in Piwnice near Toru\'n, Poland (see Bukowiecki \& Maciejewski 2008 for details). In total 478 images in $V$ and 142 in $B$ were obtained during about 56 hours of observations. The second campaign was performed between 2007 December and 2008 March with the 70/172 cm Schmidt Telescope (ST70) of the National Astronomical Observatory (NAO) at Rozhen (Bulgaria), operated by the Institute of Astronomy of the Bulgarian Academy of Sciences (see Maciejewski et al.~2008 for details). During almost 19 hours of monitoring 218 images in $V$ were acquired. About 14900 stars brighter than 19.0 mag in $V$ band were monitored in total.

One of the detected bright variables (V3 = V765 Cas) was also observed with the TSC90 in the Cassegrain mode with the Richardson spectrograph and Wright CCD camera. We obtained spectra between 3500 and 5500 {\AA} with 2 {\AA}/pix reciprocal dispersion with 600 gr/mm grating for spectral classification.

Both data sets were reduced and analysed in the way described in Bukowiecki \& Maciejewski (2008). The transformation of instrumental magnitudes into standard ones based on over 800 cluster stars which photometry was taken from Phelps \& Janes (1994). The $(B-V)$ coverage was in range between 0.25 and 2.0 mag. The residuals in observed and literature $(B-V)$ are shown in Fig.~1.

\IBVSfig{6cm}{5864-f1.eps}{The comparison of the observed photometry with the literature one.}

As a result of the analysis the following parameters were derived:\ the central coordinates $RA=01^{\mathrm{h}}19^{\mathrm{m}}38^{\mathrm{s}}$, $DEC=58\deg16'48''$, the limiting radius of $14.7\pm1.3$~arcmin, \mbox{$\log (age) = 7.40\pm0.05$}, \mbox{$E(B-V)=0.48\pm0.05$ mag}, the apparent distance modulus \mbox{$m-M=13.55\pm0.10$ mag}, and the distance of $2.6\pm0.3$ kpc. The radial density profile, plotted in Fig.~2, can be approximated with the King's formula (King 1966) with the following best-fit parameters: the central density $f_{0}=5.34\pm0.17$ stars/arcmin$^2$, the core radius $r_{\mathrm{core}}=2.43\pm0.13$ arcmin, and the density of the background stellar field $f_{\mathrm{bg}}=1.87\pm0.05$ stars/arcmin$^2$. The colour-magnitude diagram (CMD) constructed for NGC~457 with the best-fit isochrone of solar metallicity is presented in Fig.~3.

\IBVSfig{7cm}{5864-f2.eps}{The radial density profile with the best-fit King's formula. The horizontal continuous line marks the background-star-density level and the dashed ones its 3-sigma error. }

\IBVSfig{10cm}{5864-f3.eps}{The colour-magnitude diagram for NGC~457 with best-fit isochrone of solar metallicity. The open symbols denote variable stars located within the cluster's limiting radius. See text for discussion.}

As a result of our survey 29 new and 2 known variable stars were detected in the field of NGC 457. They are listed in Table~1 and their light curves are presented in Figs.~4 and 5. Twelve variables (V1--V12) are located within the cluster limiting radius. They were marked with open symbols in cluster's CMD to discuss their membership. It is clear that 4 of them, i.e. V6, V7, V9, and V10 cannot belong to the cluster for sure because they are located far from the isochrone. 


\begin{table}[t!]
\normalsize
\begin{center}
{\normalsize {\bf Table 1.} The list of variable stars detected in the field of NGC~457. $r_{\mathrm{d}}$ denotes the distance from the cluster center, $V_{\mathrm{max}}$ -- the maximal brightness in $V$ band, $\Delta V$ -- the amplitude of variation in $V$, $(B-V)$ -- the color index at the maximum of brightness, $P$ -- the period of variation, $T_{0}$ -- the epoch of minimum brightness for eclipsing systems or maximum for pulsating stars in HJD, types of variability, and cluster membership.} \vskip 2mm
\begin{tabular}{|llccccccll|}
\hline
ID & Coordinates  &  $r_{\mathrm{d}}$ & $V_{\mathrm{max}}$ & $\Delta V$ & $B-V$ & $P$ & $T_{0}$ & Type & Member.\\ 
   & J2000.0      & (')               & (mag)              & (mag)      & (mag) & (day) & 2454400+  &      & \\
\hline
V1 & 011953+581830 & 2.7 & -- & -- & -- & -- & -- & MISC & Yes\\
V2 & 011929+581340 & 3.4 & 16.33 & 0.21 & 0.97 & 0.297507 & 15.7227 & EW & No\\
V3 & 011909+581725 & 3.9 & 10.63 & 0.41 & 0.32 & 1.716280 & 15.6925 & EA & Yes\\
V4 & 012014+581435 & 5.2 & 14.54 & 0.29 & 0.68 & 0.554334 & 15.8838 & EW & No\\
V5 & 011902+581920 & 5.4 & 11.77 & 0.17 & 0.37 & -- & -- & MISC & Likely\\
V6 & 011901+581009 & 8.3 & 12.52 & 0.21 & 1.70 & -- & -- & MISC & No\\
V7 & 011852+580930 & 9.5 & 12.92 & 0.15 & 3.94 & -- & -- & MISC & No\\
V8 & 011849+582353 & 9.6 & 13.55 & 0.44 & 0.66 & 1.720430 & 39.2008 & EA & Likely\\
V9 & 011841+580756 & 11.6 & 13.39 & 0.23 & 1.83 & -- & -- & MISC & No\\
V10 & 011908+580418 & 13.1 & 14.61 & 0.26 & 1.07 & 1.824432 & 18.6319 & DCEP & No \\
V11 & 011751+581523 & 14.1 & 15.40 & 0.07 & 0.69 & 0.048419 &15.3794 & DSCT & Likely\\
V12 & 012043+582821 & 14.3 & 16.72 & 0.17 & 0.78 & 1.58797 & 18.0380 & EA & Likely\\
V13 & 011848+583138 & 16.2 & 13.97 & 0.16 & 0.85 & 4.078303 & 28.0865 & DCEP & -- \\
V14 & 011749+582430 & 16.2 & 14.73 & 0.85 & 1.09 & 0.260823 & 16.0161 & EW & -- \\
V15 & 012048+583112 & 17.1 & 15.27 & 0.20 & 1.27 & 14.44783 & 42.7258 & DCEP: & -- \\
V16 & 011757+582749 & 17.2 & 15.83 & 0.16 & 0.98 & 0.283611 & 15.8081 & EB & -- \\
V17 & 011811+583200 & 19.0 & 16.71 & 0.36 & 1.06 & 0.374246 & 16.0459 & EW & -- \\
V18 & 011733+580922 & 19.2 & 15.68 & 0.29 & 0.84 & 0.381185 & 16.0145 & EW & -- \\
V19 & 012038+583438 & 19.5 & 14.95 & 0.72 & 0.88 & 0.552955 & 16.8896 & EB & -- \\
V20 & 012023+575726 & 20.3 & 15.15 & 0.62 & 0.72 & 0.602602 & 16.7453 & RRAB & -- \\
V21 & 012155+580611 & 21.0 & 13.79 & 0.14 & 0.64 & 1.168862 & 39.5650 & CWB & -- \\
V22 & 011805+575752 & 22.6 & 14.10 & 0.48 & 0.68 & 2.571928 & 19.8162 & EA & -- \\
V23 & 011708+582827 & 22.8 & 16.54 & 0.13 & 0.66 & 0.171418 & 15.6109 & DSCT & -- \\
V24 & 012223+582408 & 22.8 & 14.76 & 0.07 & 0.67 & 0.063837 & 15.4797 & DSCT & -- \\
V25 & 012145+580121 & 22.8 & 16.21 & 0.44 & 0.91 & 0.320205 & 15.8258 & EW & -- \\
V26 & 011952+584457 & 28.2 & 15.34 & 0.11 & 0.68 & 0.888248 & 17.4587 & RR: & -- \\
V27 & 012204+583811 & 28.6 & 13.86 & 0.52 & 0.83 & 3.953425: & 18.7548 & EA & -- \\
V28 & 012038+574911 & 28.8 & 13.54 & 0.26 & 1.80 & -- & -- & MISC & -- \\
V29 & 011702+575631 & 29.0 & 15.71 & 0.12 & 1.09 & 7.845039 & 42.1188 & DCEP: & --\\
V30 & 011917+574559 & 30.9 & 15.70 & 0.13 & 0.77 & 0.194762 & 15.6996 & DSCT & -- \\
V31 & 011712+584958 & 38.2 & 13.57 & 0.25 & 1.15 & -- & -- & MISC & -- \\
\hline
\multicolumn{10}{|l|}{Remarks: V1 = V466 Cas, V3 = V765 Cas} \\
\hline
\end{tabular}
\end{center}
\end{table}

\AladinFindingChart{5864-t1}
\IBVSecomment{The stars in Table 1 viewed with CDS Aladin}


V1 is a red and bright evolved star known as V466 Cas -- an irregular pulsating variable. The star was found to be saturated in our short $V$ band exposures, however its light curve was recorded in the $B$ band database and plotted in Fig.~4. Its $V$ magnitude and the $(B-V)$ colour index were taken from the SIMBAD database. The star and NGC~457 share the common proper motions (Perryman et al.~1997, Loktin \& Beshenov 2003) what allows to conclude that V1 (V466 Cas) is cluster's member.

V2 is a faint contact system located near the isochrone. Assuming it belongs to the cluster, its absolute magnitude is $M_{\mathrm{V}}=2.7$ mag. The same quantity calculated from the empirical formula of Ruci\'nski \& Duerbeck (1997) is 3.8 mag. Therefore, we conclude that V2 is a background star.

V3 is known as V765 Cas -- an eclipsing system of EB type of spectral type B5. Our photometry clearly indicates that the variable is de facto a short period Algol-type system with a typical shape of minima and unequal brightness near the maxima. The variable is situated near the isochrone thus it can be treated as cluster's member. Additionally, the star was observed spectroscopically with the TSC90 in the Cassegrain mode to redetermine its spectral type. The spectrum is plotted in Fig.~6 where spectral lines that were used for classification are marked with arrows. The ratios of HeI[$\lambda$4026]/HI[$\lambda$4340] and HeI[$\lambda$4471]/HI[$\lambda$4340] were considered. As a result the spectral type of V3 (V765 Cas) was found to be slightly earlier, i.e. B2.5.

The light-curve variability of V4 indicates that it is a contact system. Assuming it belongs to the cluster, its absolute magnitude is $M_{\mathrm{V}}=0.9$ mag. The same quantity calculated from the empirical formula of Ruci\'nski \& Duerbeck (1997) is much greater, i.e., 1.7~mag. Therefore, we conclude that the membership of V4 is unlikely.

V5 was found to be a blue pulsating variable of unresolved type, revealing brightness changes on long-time scale. It is situated in the bright part of cluster's CMD in the area of SPB variables, thus its membership is likely.

V8 is a detached eclipsing system. Its location in the CMD suggests that it can belong to the cluster.

V11 is a short-period pulsating variable of $\delta$~Scuti type. Assuming it belongs to the cluster, its absolute magnitude is $M_{\mathrm{V}}=1.8$ mag -- a typical value for variables of that type. This suggests that membership of V11 is likely.

V12 was classified as a faint detached eclipsing system. It is located near the isochrone, thus the variable can be treated as cluster's member.

Concluding, 6 variables detected in the field of NGC~457 are unquestionable or likely cluster members. The cluster is found to be rich in eclipsing systems represented by 3 detached binaries. More interestingly, brightness of the systems decreases with distance from the cluster centre. Assuming an eclipsing binary belonging to the cluster, its maximum brightness can be interpreted as a rough approximation of its total mass. As a result of the mass segregation, more massive (i.e. brighter) systems are expected to occupy the central part of a cluster while the less massive (i.e. fainter) ones -- the outer region (e.g. Lamers et al.~2006 and references therein).

The original photometric data are available on the survey's web site: \\ 
{\tt http://www.astri.uni.torun.pl/\~{}gm/OCS}.


\textit{Acknowledgments:}  This paper is a result of PAN/BAN exchange and joint research project \textit{Spectral and photometric studies of variable stars}. The research is supported by UMK grant 411-A and Grants VU-F 201/06 and VU-NZ-01/06 of the Bulgarian Science Foundation. The research has made use of the WEBDA and SIMBAD data bases. 


\references

Bukowiecki, {\L}., Maciejewski, G. 2008, IBVS, 5857

Dias, W.~S., Alessi, B.~S., Moitinho, A., Lepine, J.~R.~D. 2002, {\it A\&A}, {\bf 389}, 871

King~I. 1966, {\it AJ}, {\bf 71}, 64

Lamers, H. J. G. L. M., Anders, P., de Grijs, R. 2006, {\it A\&A}, {\bf 452}, 131

Loktin, A. V., Beshenov, G. V. 2003, {\it ARep}, {\bf 47}, 6

Maciejewski, G., Georgiev, Ts., Niedzielski, A. 2008, {\it Astron. Nachrichten}, {\bf 329}, 387

Mermilliod, J.~C. 1996, {\it ASP Conf. Ser.}, {\bf 90}, 475, in The Origins, Evolution 
and Destinies of Binary Stars in Cluster, ed. E. F. Milone \& J.-C. Mermilliod, 

Perryman, M. A. C., Lindegren, L., Kovalevsky, J., et al. 1997, {\it A\&A}, {\bf 323}, L49

Phelps, R. L., Janes, K. A. 1994, {\it ApJS}, {\bf 90}, 31

Ruci\'nski, S.~M., Duerbeck, H.~W. 1997, {\it PASP}, {\bf 109}, 1340

\endreferences



\IBVSfig{22.5cm}{5864-f4.eps}{Light curves of variable stars discovered in the field of NGC~457.}

\IBVSfig{13cm}{5864-f5.eps}{Light curves of variable stars discovered in the field of NGC~457.}

\IBVSfig{6cm}{5864-f6.eps}{The optical spectrum obtained for V3 (V765~Cas) in the blue. Spectral lines that were used for classification are marked with arrows.}

\IBVSedata{5864-t2.txt}
\IBVSdataKey{5864-t2.txt}{NGC 457}{starlist}
\IBVSedata{5864-t3.txt}
\IBVSdataKey{5864-t3.txt}{Cl* NGC 457 MBB V1}{photometry}
\IBVSedata{5864-t4.txt}
\IBVSdataKey{5864-t4.txt}{Cl* NGC 457 MBB V2}{photometry}
\IBVSedata{5864-t3.txt}
\IBVSdataKey{5864-t5.txt}{Cl* NGC 457 MBB V3}{photometry}
\IBVSedata{5864-t6.txt}
\IBVSdataKey{5864-t6.txt}{Cl* NGC 457 MBB V4}{photometry}
\IBVSedata{5864-t7.txt}
\IBVSdataKey{5864-t7.txt}{Cl* NGC 457 MBB V5}{photometry}
\IBVSedata{5864-t8.txt}
\IBVSdataKey{5864-t8.txt}{Cl* NGC 457 MBB V6}{photometry}
\IBVSedata{5864-t9.txt}
\IBVSdataKey{5864-t9.txt}{Cl* NGC 457 MBB V7}{photometry}
\IBVSedata{5864-t10.txt}
\IBVSdataKey{5864-t10.txt}{Cl* NGC 457 MBB V8}{photometry}
\IBVSedata{5864-t11.txt}
\IBVSdataKey{5864-t11.txt}{Cl* NGC 457 MBB V9}{photometry}
\IBVSedata{5864-t12.txt}
\IBVSdataKey{5864-t12.txt}{Cl* NGC 457 MBB V10}{photometry}
\IBVSedata{5864-t13.txt}
\IBVSdataKey{5864-t13.txt}{Cl* NGC 457 MBB V11}{photometry}
\IBVSedata{5864-t14.txt}
\IBVSdataKey{5864-t14.txt}{Cl* NGC 457 MBB V12}{photometry}
\IBVSedata{5864-t15.txt}
\IBVSdataKey{5864-t15.txt}{Cl* NGC 457 MBB V13}{photometry}
\IBVSedata{5864-t16.txt}
\IBVSdataKey{5864-t16.txt}{Cl* NGC 457 MBB V14}{photometry}
\IBVSedata{5864-t17.txt}
\IBVSdataKey{5864-t17.txt}{Cl* NGC 457 MBB V15}{photometry}
\IBVSedata{5864-t18.txt}
\IBVSdataKey{5864-t18.txt}{Cl* NGC 457 MBB V16}{photometry}
\IBVSedata{5864-t19.txt}
\IBVSdataKey{5864-t19.txt}{Cl* NGC 457 MBB V17}{photometry}
\IBVSedata{5864-t20.txt}
\IBVSdataKey{5864-t20.txt}{Cl* NGC 457 MBB V18}{photometry}
\IBVSedata{5864-t21.txt}
\IBVSdataKey{5864-t21.txt}{Cl* NGC 457 MBB V19}{photometry}
\IBVSedata{5864-t22.txt}
\IBVSdataKey{5864-t22.txt}{Cl* NGC 457 MBB V20}{photometry}
\IBVSedata{5864-t23.txt}
\IBVSdataKey{5864-t23.txt}{Cl* NGC 457 MBB V21}{photometry}
\IBVSedata{5864-t24.txt}
\IBVSdataKey{5864-t24.txt}{Cl* NGC 457 MBB V22}{photometry}
\IBVSedata{5864-t25.txt}
\IBVSdataKey{5864-t25.txt}{Cl* NGC 457 MBB V23}{photometry}
\IBVSedata{5864-t26.txt}
\IBVSdataKey{5864-t26.txt}{Cl* NGC 457 MBB V24}{photometry}
\IBVSedata{5864-t27.txt}
\IBVSdataKey{5864-t27.txt}{Cl* NGC 457 MBB V25}{photometry}
\IBVSedata{5864-t28.txt}
\IBVSdataKey{5864-t28.txt}{Cl* NGC 457 MBB V26}{photometry}
\IBVSedata{5864-t29.txt}
\IBVSdataKey{5864-t29.txt}{Cl* NGC 457 MBB V27}{photometry}
\IBVSedata{5864-t30.txt}
\IBVSdataKey{5864-t30.txt}{Cl* NGC 457 MBB V28}{photometry}
\IBVSedata{5864-t31.txt}
\IBVSdataKey{5864-t31.txt}{Cl* NGC 457 MBB V29}{photometry}
\IBVSedata{5864-t32.txt}
\IBVSdataKey{5864-t32.txt}{Cl* NGC 457 MBB V30}{photometry}
\IBVSedata{5864-t33.txt}
\IBVSdataKey{5864-t33.txt}{Cl* NGC 457 MBB V31}{photometry}


\end{document}

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